The present invention relates to a steel wire for use in the manufacture of a chassis or suspension spring which has high sag resistance. The invention also relates to a method for producing the same.
Careful consideration is given to such factors as sag resistance, relaxation and the like in producing a steel wire for a suspension spring. (In this specification, the term "sag" is used to mean permanent change of form of a coil spring due to repeated or excess stress, resulting in changes in the free height thereof.) Conventional steel wires of a type which have often been utilized in producing such a spring include those produced in accordance with SUP 6 to 7 and SAE 9254.
With the growing demands in recent years for fuel savings, there has been a need for lightweight automotive vehicles. For these reasons, it is necessary to provide a steel wire which is excellent in sag resistance. More particularly, development of such steel wire permits improvements in design stress, and as a result, more lightweight steel wires may be produced. Attempts have hitherto been made to impart high strength to a steel wire by means of a great variety of heat treatments for the purpose of improving sag resistance. However, if the steel wire is excessively strengthened in such a manner, this results in a decrease of the value of ductility factor such as elongation and reduction of area. Due to such excessive strength, the steel wire is liable to be damaged during processing or use, particularly if there are present fine defects on the surface of the steel wire and internal defects therein.
Under these circumstances, as a result of research, the inventors have succeeded in providing a practical and effective steel wire for springs which improves sag resistance without increasing the strength of the wire.